Accelerator for fishing jars

ABSTRACT

An accelerator for fishing jars for removing stuck objects from a well bore located above a drill collar string, the jar being located between a fishing tool and the drill collar string. A resiliently compressible body of rubber-like material is disposed in a pressure chamber in the accelerator containing a lubricating oil pressure transfer medium to apply pressure to all surfaces of the compressible body and reduce its volume when the fishing string is tensioned, the compressible body acting as a spring on expansion to accelerate the drill collar mass when the jar is released to produce the jarring action. The resilient material has a high impedance to the transmission of sound to improve jarring efficiency.

States atent 1 1 [111 Q erryman 1 my 29, 11973 [54] AQCELERATOR FORFISHING JARS 3,606,297 9 1971 Webb 175 321 x l A k 7 [75] Inventor:William 0. Berryman, Houston, 3642069 I 2/ 972 d m 1 5/296 X I PrimaryExaminer-David H. Brown [73] Assigneei Baker Oil Tools, Inc., LosAngeles, Attorney-Bernard Kl'iegel Calif.

[57] ABSTRACT [22] Filed: Mar. 15, 1972 l An accelerator for fishmg jarsfor removing stuck obl l PP jects from a well bore located above a drillcollar string, the jar being located between a fishing tool and 521 US.Cl. .......-......17s/299, 175/321, 166/178 i string- 5* @EPcgmPressible 51 m. u. ..,.E2lb 1/06, E2lb 1/10, E2lb 23/00 F" e mater F?m 'P 'F chamber in the accelerator contalrnng a lubr1cat1ng 011 [58]Field of Search ..l66/l78, 175/293, 175/296 297 299 300 302 303 34pressure transfer medlum to apply pressure to all surfaces of thecompressible body and reduce its volume when the fishing string istensioned, the compressible [561' Refgrences Cited body acting as aspring on expansion to accelerate the UNITED STATES ATENTS drill collarmass when the jar is released to produce the arrmg act1on The res1l1entmatenal has a h1gh 1m- 3,472,326 10/1969 Sutliffet a]. ..l75/32l Xpedance to the transmission of sQund to improve jarl Sutliffet a] ringefficiency 3,539,026 11/1970 Sutliffet al.,. .....175/299.

3/1971 Slator ..l75/297 l7 Claimsyfi Drawing Figures /////7"r/// /77 I lJ y/ 1 it PATENTED MAY 2 9 I973 SHEET 1 UF 2 PATENTEB MAY 2 91975 SHEET2 BF 2 AC'CELERATUR F FISHING JARS ferred to as the fishing or runningstring, in a position above the fishing tool, which may be a spear orovershot adapted to engageand grip the fish, enabling the fish to bepulled from the well bore upon pulling the fishing string. The jar isemployed for the purpose of applying hammer blows tending to release thestuck fish while the fishing string is under tension. Jars of thehydraulic type, in general, comprise a pressure chamber containing aquantity of hydraulic fluid which is compressed when an upward strain ortension is applied to the running string. When the compressed hydraulicfluid is allowed to rapidly by-pass a compression piston, the tension ofthe running string causes a hammer to impact against an anvil to apply ajarring or hammer blow tending to release or remove the stuck object orfish.

The magnitude of the hammer blow is a function of the tension applied tothe fishing string and the mass of the fishing string, which must beaccelerated upwardly. The fishing string ordinarily includes heavy drillcollars between the fishing tool and the drill pipe. To be effective,the tension in the fishing string must rapidly accelerate the drillcollars, since the hammer blow is determined by the mass and velocity ofthe drill collars at the moment of impact of the hammer against theanvil. Thus, various accelerator devices haveevolved which are adaptedto provide a spring force acting between the jarring tool and thefishing string to assist in the acceleration of the upward movement ofthe jarring hammer by accelerating upward movement of the drill collars.Such accelerators have utilized a compressible gas in a compressionchamber or a compressibleliquid, such as oil, to provide a spring forcecapable of accelerating the upward motion of the fishing string. Toenable sufficient stroke of the accelerator, the compressible oil isusually a silicone oil having, as compared with better lubricating oil,a low bulk modulus, since a lubrieating oil, in general, has such a highbulk modulus that a compression chamber of excessive length is required.

Moreover, in the absence of an accelerator, the acceleration of thedrill collars is affected by the upward travel of the wave front,instantaneously produced upon release of the jar, which travels upwardlyat the speed of sound through the drill collars and is partiallyreflected downwardly at the interface of the drill collars with thedrill pipe above the drill collars through a number of cycles before thehammer of the jar impacts with the anvil. The acceleration of the drillcollars is accentuated with the upward phase of each cycle of the wavefront. Since fishing operations involve the circulation of fluiddownwardly-through the fishing string of pipe, it is inherent that thepipe string is of substantial area and much of the sonic wave front istransmitted from the drill collars to the drill pipe, therebydiminishing the reflected wave, which moves downwardlyv through thedrill collars and .then turns around and moves upwardly to assist inaccelerating the drill collar mass upwardly before the hammer strikesthe anvil in the jar.

The present invention involves the provision of an accelerator which hasa compression chamber containing a resiliently compressible rubber orrubber-like material having a low bulk modulus and a high rate ofrecovery, and provides substantial spring force in a small space foraccelerating upward movement of a drill collar mass that produces thejarring force in a fishing jar.

In accordance with the invention, the resiliently compressible materialhas a low product of mass density times speed of sound, so that therelatively high impedance to the transmission of sound results in areflected wave of proportionally high value when the wave front producedat the time of release of the jar travels upwardly through the drillcollars to the accelerator.

More particularly, the present invention involves utilizing arubber-like material in the compression chamber of the accelerator whichhas a low bulk modulus, such as silicone rubber, that permits a smallvolume of the material to be used in the compression chamber for thedesired energy to be stored in the material and stroke of theaccelerator, thereby reducing the overall length of the compressionchamber and of the accelerator itself. By way of example, siliconerubber has a bulk modulus of 150,000 p.s.i., as compared with theaverage rubber which has a bulk modulus of 345,000 p.s.i., the siliconerubber being far more compressible than the average rubber and,consequently, being far more expansive than the average rubber when bothtypes of rubber are subjected to the same compressive force.Accordingly, for the same compressive force and extent of expansion, thecompression chamber with silicone rubber therein can be much shorterthan with an average rubber therein. The rubber material is disposed inthe chamber and surrounded by a fluid pressure transfer medium, whichapplies pressure to all surfaces of the rubber material to resilientlycompress or deform it. Since the fluid is not relied upon to provide thespring action, the fluid may be a good lubricating oil to preventgalling of the telescopic parts of the accelerator, and the hazards ofpressure filling of gas accelerators are avoided.

This invention possesses many other advantages, and has other purposeswhich may be made more clearly apparent from a consideration of a formin which it may be embodied. This form is shown in the drawingsaccompanying and forming part of the present specification. It will nowbe described in detail, for the purpose of illustrating the generalprinciples of the invention; but it is to be understood that such adetailed description is not to be taken in a limiting sense.

Referring to the drawings:

FIGS. la through He together constitute a longitudinal quarter sectionthrough an accelerator device made in accordance with the invention,FKGS. lb through lle being, respectively, successive downward extensionsof HG. la; and

FIG. 2 is a view showing a fishing string, including a jar and anaccelerator in a well bore and connected to a fish by a fishing tool.

As seen in the drawings, the accelerator A of the invention comprises anelongated and telescopic assembly of an inner tubular body or mandrel land an outer tubular body or housing 2. At its upper end, the mandrel lis provided with an internally threaded box 3 adapted to receive theexternally threaded pin a at the lower end of an upwardly extendedsection of drill pipe P included in the fishing string S, which isadapted to extend to the top of a well bore W so that tension can beapplied to the fishing string to operate ajar J for applying hammerblows to a fishing tool T, such as a spear or an overshot, to a pipe orother fish F, stuck in the well bore W. Typically, such jars are soconstructed that they are adapted to sustain a predetermined pull andthey then release to enable a hammer to strike an anvil. By way ofexample, the jar may be of the downhole adjustable type shown inco-pending application Ser. No. 234,737, filed Mar. 15 1972, forDown-Hole Adjustable Hydraulic Fishing J ar. A string of drill collars Care interposed between the accelerator A and the jar J to provide aheavy mass to be accelerated upwardly by energy stored in the fishingstring S and in the accelerator A, as is well known. The lower end ofthe accelerator housing 2 has a threaded pin 5 engaged in a threaded box6 at the upper end of the drill collars C.

More particularly, the body or mandrel 1 comprises an upper tubular bodysection 7 internally threaded at 8 to receive the upper, externallythreaded end of a lower body or mandrel section 9. The upper end portion10 of the mandrel section 9 has an external cylindrical wall 11 opposedby a complemental cylindrical wall 12 on the mandrel section 7. Asuitable ring seal 13 is provided between the walls 11 and 12 to preventleakage of fluid circulated through the accelerator during a fishingoperation.

The outer body or housing 2 is shown as comprising an upper tubularhousing section 14, having a lower threaded connection 15 with the neck16 of an annular cylinder head 17. This cylinder head 17 also has anexternally threaded skirt 18 connected to the internally threaded upperend of an elongated cylinder or outer body member 19, the lower end ofwhich is internally threaded at 20 to receive the externally threadedupper end portion 21 of a lower body member and connector 22 having thethreaded pin 5 thereon connected to the drill collars C.

The inner body member or mandrel section 9 and the outer body member orcylinder 19 define therebetween an elongated, annular chamber 23, closedat its upper end by upper seal means 24 and at its lower end by lowerseal means 25. The upper seal means 24 comprises a pair of packing orseal rings 26 disposed between the outside wall of the mandrel section 9and an opposing cylindrical wall 27 defining a bore 28 in the cylinderhead 17. A sleeve 29 extends upwardly in the bore 28 to form with a sealback-up member 30, at the top of the bore 28, an annular internal groovein which the seal rings 26 are disposed. At its lower end, the sleeve 29is threaded into the internally threaded lower extremity 31 of thecylinder head skirt 18, the skirt 18 having a lower flange 32 abuttingbeneath the skirt 18. Also, the chamber 23 is sealed by a seal ring 33between opposing sealing walls 34 and 35 on the skirt 18 and thecylinder 19. The sleeve 29 defines with the mandrel section 9 aclearance 36 which communicates, through a suitable number of radialports 37 in the sleeve 29, with a bleed port 38 in which is a bleedervalve 39 held closed by a plug 40. At a suitable location below thecylinder head 17, the housing 2 has an inlet or fill opening 41 and plug42, whereby the chamber 32 may be filled with hydraulic pressuretransfer fluid, and air can bleed off through the bleed opening 38.

The lower seal means comprises an annular piston 43 reciprocable in thechamber 23 between opposed,

cylindrical walls 44 and 45 of the inner and outer bodies 1 and 2. Thispiston has inner seal rings 46 slidably and sealingly engaging thecylindrical wall 44 and outer seal rings 47 slidably and sealinglyengaging the cylindrical wall 45. Below the piston 43, the chamber 23communicates with the fluid passage through the tool via an annularspace 48a which opens into the lower body section or connector sub 22.Therefore, the piston 43 is a compensating piston to balance pressure inthe chamber with pressure in the running or fishing string of pipe.

In the embodiment shown, the cylindrical wall 45 engaged by thecompensating piston 43 is provided by a sleeve or liner 48 disposedwithin the housing section 19 and seating at its lower end between acylindrical wall 49 of an end flange 50 on the neck 21 of the connector22 and the inside wall of the housing section 19, a seal ring 51 beingprovided about the end flange 50 to seal against the liner 48. At itsupper end, the liner 48 provides a radial stop shoulder 52 whichcooperates with compression piston means 53, now to be described, tolimit downward movement of the latter.

The compression piston means 53 are operable to compress fluid in thechamber 23 upon telescopic extension of the bodies 1 and 2, when thefishing string S is tensioned. When the jar is operated and the bodies 1and 2 are telescopically contracted, the piston means 53 allow theby-pass of fluid in the chamber to again enable equalization of internaland external pressures by the compensating piston 43.

More particularly, the compression piston means 53 comprises an annularpiston body 54 slidably engageable within the cylindrical inner wall 55of the housing cylinder 19 and having an inside diameter larger than thecylindrical wall 56 of the mandrel section 9, to provide an annular flowpassage 57 between the piston 54 and the mandrel section 9. About itsouter periphery, the piston 54 has a pair of seal rings 58 disposedbetween a back-up ring 59 and an opposed ring 60 retained on the pistonby a lock ring 61, so that the seal rings 58 are in sliding and sealingengagement with the cylinder wall 55. A stop shoulder 62 on the piston54 is engageable with the upper end 52'of the liner 48 to limit downwardmovement of the piston 54.

A lifter or shoulder 63 for moving the piston 54 upwardly in the chamber23 is provided on the mandrel section 9, to be spaced below the lowerend of the piston 54 when the bodies 1 and 2 are telescopicallycontracted, as shown. Above the shoulder 63, the mandrel section 9 has acylindrical surface 64 and a seal ring 65, constituting a valvesealingly engageable in a bore 67 in the lower portion of the piston 54when the mandrel 1 is elevated with respect to the housing 2, to preventfluid from bypassing the piston 54 through the annular space 57, andcausing telescopic extension of the mandrel 1 and housing 2 to effectcompression of the medium within the chamber 23 as the piston 54 isshifted upwardly in the cylinder 19. Return downward movement of thepiston 54 is assured by an abutment above the piston on the mandrelsection 9, in the illustrated form of a stop ring 68 held against upwardmovement by a lock ring 69, the stop ring 68 having flow passages 70 toallow fluid to by-pass the piston 54 as it is being cooperative portionsof the mandrel 1 and the housing 2. As shown, the upper end of thehousing section M v has a downwardly facing shoulder 71 formed on a head72 and companion grooves 75 and ribs 76 extending longitudinally of themandrel section 7 at circumferentially spaced locations. Adjacent to thelower end of the mandrel section 7 is a stop collar 77, the upper end 7%of which is opposed to the mandrel shoulder 71 to engage with the latterupon upward movement of the mandrel l with respect to the housing 2.This stop collar is threaded at 79 onto the mandrel section 7 and lockedin place by a cap screw. b threaded into the mandrel section 7 andhaving its head disposed in a lateral opening 811 in the stop collar 71.

As thus far described, the accelerator A is operable when the running orfishing pipe string S is tensioned to apply tension to the jar J, tocompress the medium in the chamber 23 to an extent determined by themagnitude of the tension applied to the fishing string to release thejar J, as is well known, until the jar releases and allows the drillcollars C to move upwardly, accelerated by stored energy in thetensioned fishing string S and in the accelerator A. The force of thejarring action is, generally, determined by the mass of the moving drillcollars and the drill collar velocity at impact of the jarring hammerwith the anvil in the jar apparatus.

The present invention involves the utilization of a compressible orresiliently deformable material in the chamber 23 which is, within theinherently small available annular space, capable of being reduced involume and storing substantial energy useful, upon release of the jar J,to assist the fishing string tension in accelerating the upward motionof the drill collars C, to increase the effective velocity and theresultant force of the hammer blow. in accomplishing this purpose, thepreferred material is an elastomer, devoid of cavities, and having a lowbulk modulus, such as silicone rubber, the bulk modulus of which is150,000 p.s.i. In addition, a material is preferable which has a highimpedance to sound transmission, so that the jarring effectiveness isfurther enhanced.

Thus, a body 22 of rubber or rubber-like material, which is resilientlydeformable or compressible to reduce its volume and which rapidlyrecovers when deforming pressure is released, is disposed in thecompression chamber 23to be deformed therein under pressure when theaccelerator is telescopically extended, and provide a high rateresilient force to cause telescopic contraction of the accelerator whenthe jar is released.

As shown, the resilient body 82 is of cylindrical form and is disposedbetween opposing cylindrical walls 23 and 84 defining the compressionchamber 23, the body 02 being much shorter than the distance between theflange 32 and the piston 54 when resting on the shoulder 52. The radialthickness of'the body $2 is less than the radial distance between thewalls 03 and 04 and therefore, the fluid or hydraulic oil in the chamber23 can contact both the inner and outer cylindrical walls of the bodyb2. In addition, the resilient body $2 is exposed at its ends to thepressure of fluid or oil in the chamber 23- and fills sufficient spacein the chamber 23 as to constitute the effective energy storing means.Thus, pressure applied to the resilient body upon upward movement of thecompression piston means 53 is applied through the liquid in the chamber23 to all surfaces equally by the fluid pressure transfer medium orliquid, to compress the resilient body 82 in all directions. Preferably,the body 82 is formed of a solid rubber or rubber-like material, such assilicone rubber, which is devoid of air pockets or cells, the materialhaving a high rate of recovery from compression and capable of storingmore energy in a given space than can be stored by hydraulic fluid oroil, thereby producing a longer stroke to assist in accelerating thedrill collars C when the jar releases. Since the fluid medium in thechamber 23 does not store the substantial energy which assists inaccelerating the drill collars, the fluid is selected from oils whichhave good lubricating qualities to reduce friction.

in addition, rubber or rubber-like materials, such as silicone rubber,have a high impedance to sound, as compared with oil or hydraulic fluid.Therefore, the transmission of sound waves through the acceleratorassembly from the drill collars to the drill pipe is impeded by the useof the resilient, energy storing body 82, and the accelerator is moreeffective for a given length. Thus, when the upwardly moving wave fronttravelling through the drill collars, instantaneously caused in thedrill collars when the jar is released and upward acceleration of thedrill collars commences, reaches the accelerator A, the increasedimpedance of the accelerator caused by the resilient body 82, results ina reduction in the transmission of the upwardly moving wave frontthrough the accelerator, and more energy is reflected downwardly. Inthis connection, the value of the reflected wave is determined by thearea of the accelerator as compared with the area of the drill collarsand the velocity of sound through them, and as the value of thereflected wave is increased, the lower the product of mass density ofthe medium in the chamber 23, including the resilient body b2, times thespeed of sound. The high impedance also'reduces the net reflection ofthe wave front at the connection between the accelerator A and the drillpipe 1?, which could, otherwise, tend to damp out the useful wave fronttravelling upwardly in the drill collars C. The period of time requiredfor the jarring action to occur, after release of the jar, is sufficientto enable the reflected wave front to travel downwardly through thedrill collars C from the interface with the accelerator and again returnupwardly through a number of cycles, and at each turn around of the wavefront at the bottom of the drill collars, the acceleration of the drillcollars is increased, to thereby increase the effective hammer blow ofthe jar, which is determined by the mass and velocity of the drillcollars at impact of the usual hammer and anvil.

Summarizing the operation of the invention, as illustrated in FIG. 2,the fishing string is lowered in the well bore W, with the drill pipe Pconnected to the upper end of the accelerator A, below which are thedrill collars C and the fishing tool F. The fishing tool F is connectedto the fish or stuck pipe in the usual manner, and, as desired ornecessary, fluid is circulated downwardly through the fishing string.

ing 2, and the compressible medium in the pressure chamber 23 iscompressed by the compression piston means 53. The compressive forceapplied to the fluid or oil in the chamber 23 is transferred or appliedto all surfaces of the compressible body 82, causing it to re silientlyreduce in volume until the maximum tension is applied to the fishingstring, or until the stop collar 77 on the mandrel l engages the stopshoulder 71, to limit further compression of the fluid medium and therubber body 82 in the chamber 23. The use of the body 82, instead ofoil, to store the energy enables the accelerator assembly to produce,relatively, a longer effective stroke for the length of the tool and themagnitude of the compressive force, as well as enabling the use of aselected lubricating oil.

When the jar releases and the mandrel 1 and the drill collars C aremoved upwardly by energy stored in the fishing string and theaccelerator, the energy stored in the rubber body 82 maintainsan upwardforce on the accelerator housing 2 throughout the entire jarring stroke,until the jarring hammer strikes the anvil, as is customary in fishingjars.

Inasmuch as the wave front originating at the bottom of the drillcollars and travelling upwardly therethrough at the speed of sound istransmitted through the accelerator to an extent determined by the areaand material of the accelerator, and a proportional reflected wavereturns downwardly through the drill collars, then turns around andassists in accelerating the drill collars, the invention provides anaccelerator which has a high impedance to sound by utilizing as thecompressible material or body 82 a high impedance material, such asrubber.

I claim:

1. In an accelerator for fishing jars connectable to a fishing pipestring and to a drill collar string, the mass of which moves a jarringhammer into engagement with an anvil: inner and outer telescopic bodies,one of said bodies having means for connecting it to the fishing pipestring, the other of said bodies having means for connecting it to thedrill collar string, means between said bodies defining a compressionchamber containing a compressible medium, compression piston meansoperable upon telescopic extension of said bodies to compress saidmedium to store energy tending to effect telescopic contraction of saidbodies and accelerate said drill collar string, said compressible mediumcomprising a body of resiliently compressible material and a fluidpressure transfer material in engagement therewith, the volume of saidcompressible material being decreased on its compression by said fluidpressure transfer material pressurized by relative movement between saidtelescopic bodies.

2. An accelerator as defined in claim 1, said body being composed ofrubber-like material having a low bulk modulus.

3. An accelerator as defined in claim 1, said body being composed ofsilicone rubber.

4. An accelerator as defined in claim 1, said body having a highimpedance to the transmission of sound therethrough as compared to thefluid pressure transfer material.

5. An accelerator as defined in claim 1, said fluid pressure transfermaterial comprising a lubricating oil.

6. An accelerator as defined in claim 1, said fluid pressure transfermaterial comprising a lubricating oil and said resiliently compressiblebody comprising rubber-like material.

7. An accelerator as defined in claim 1, said fluid pressure transfermaterial comprising a lubricating oil and said resiliently compressiblebody comprising silicone rubber.

8. An accelerator as defined in claim 1, said body having thecharacteristic that the product of its mass density times the speed ofsound therethrough is lower than the product of the mass density timesthe speed of sound through said fluid pressure transfer material.

9 An accelerator as defined in claim 1, wherein said compression chamberis an annular chamber defined by opposing cylindrical walls of saidtelescopic bodies, said resiliently compressible body comprising acylinder of rubber-like material disposed between said cylindrical wallsand substantially filling said chamber.

10. An accelerator as defined in claim 1, wherein said compressionchamber is an annular chamber defined by opposing cylindrical walls ofsaid telescopic bodies, said resiliently compressible body comprising acylinder of rubber-like material disposed between said cylindrical wallsand having a radial thickness less than the radial distance between saidopposing walls.

11. An accelerator as defined in claim 1, wherein said compressionchamber is an annular chamber defined by opposing cylindrical walls ofsaid telescopic bodies, said resiliently compressible body comprising acylinder of rubber-like material disposed between said cylindrical wallsand having a radial thickness less than the radial distance between saidopposing walls, and mengageable stop means on said bodies for limitingtelescopic extension of said telescopic bodies.

12. Apparatus for jarring a fish stuck in a well bore: comprisingfishing tool means engageable with said fish, jarring means connected tosaid fishing tool means, a length of drill collars connected to saidjarring means, accelerator means connected to said drill collars andhaving means for connecting said accelerator means to a pipe string bywhich said fishing tool means, said jarring means, said length of' drillcollars and said accelerator means are adapted to be lowered in the wellbore to engage said fishing tool means with said fish, said acceleratormeans being constructed so that the wave front created upon actuationand release of said jarring means in response to the tension in saidaccelerator means and said length of drill collars is reflecteddownwardly by said accelerator means, said accelerator means including aresiliently compressible elastomeric medium, and means for compressingsaid medium upon tensioning said accelerator means.

13. Apparatus for jarring a fish stuck in a well bore as defined inclaim 12, wherein said means for compressing said medium comprises acompression chamber in said accelerator containing said elastomericmedium and a pressure transfer fluid for applying pressure to saidmedium.

14. Apparatus for jarring a fish stuck in a well bore as defined inclaim 12, wherein said means for compressing said medium comprises acompression chamber in said accelerator containing said elastomericmedium and a pressure transfer fluid for applying pressure to saidmedium, said medium being composed of rubber having a low bulk modulus.

15. Apparatus for jarring a fish stuck in a well bore as defined inclaim 12, wherein said means for compressing said medium comprises acompression chamber in said accelerator containing said elastomericmedium and a pressure transfer fluid for applying pressure to saidmedium, said medium being composed of rubber having a low bulk modulusand said fluid being a lubricating oil.

16. Apparatus for jarring a fish stuck in a well bore as defined inclaim 12, wherein said means for compressing said medium comprises acompression chamber in said accelerator containing said elasto'rnericmedium and a pressure transfer fluid for applying pressure to saidmedium, said medium being composed of silicone rubber, and said pressuretransfer fluid being a lubricating oil.

1. In an accelerator for fishing jars connectable to a fishing pipestring and to a drill collar string, the mass of which moves a jarringhammer into engagement with an anvil: inner and outer telescopic bodies,one of said bodies having means for connecting it to the fishing pipestring, the other of said bodies having means for connecting it to thedrill collar string, means between said bodies defining a compressionchamber containing a compressible medium, compression piston meansoperable upon telescopic extension of said bodies to compress saidmedium to store energy tending to effect telescopic contraction of saidbodies and accelerate said drill collar string, said compressible mediumcomprising a body of resiliently compressible material and a fluidpressure transfer material in engagement therewith, the volume of saidcompressible material being decreased on its compression by said fluidpressure transfer material pressurized by relative movement between saidtelescopic bodies.
 2. An accelerator as defined in claim 1, said bodybeing composed of rubber-like material having a low bulk modulus.
 3. Anaccelerator as defined in claim 1, said body being composed of siliconerubber.
 4. An accelerator as defined in claim 1, said body having a highimpedance to the transmission of sound therethrough as compared to thefluid pressure transfer material.
 5. An accelerator as defined in claim1, said fluid pressure transfer material comprising a lubricating oil.6. An accelerator as defined in claim 1, said fluid pressure transfermaterial comprising a lubricating oil and said resiliently compressiblebody comprising rubber-like material.
 7. An accelerator as defined inclaim 1, said fluid pressure transfer material comprising a lubricatingoil and said resiliently compressible body comprising silicone rubber.8. An accelerator as defined in claim 1, said body having thecharacteristic that the product of its mass density times the speed ofsound therethrough is lower than the product of the mass density timesthe speed of sound through said fluid pressure transfer material.
 9. Anaccelerator as defined in claim 1, wherein said compression chamber isan annular chamber defined by opposing cylindrical walls of saidtelescopic bodies, said resiliently compressible body comprising acylinder of rubber-like material disposed between said cylindrical wallsand substantially filling said chamber.
 10. An accelerator as defined inclaim 1, wherein said compression chamber is an annular chamber definedby opposing cylindrical walls of said telescopic bodies, saidresiliently compressible body comprising a cylinder of rubber-likematerial disposed between said cylindrical walls and having a radialthickness less than the radial distance between said opposing walls. 11.An accelerator as defined in claim 1, wherein said compression chamberis an annular chamber defined by opposing cylindrical walls of saidtelescopic bodies, said resiliently compressible body comprising acylinder of rubber-like material disposed between said cylindrical wallsand having a radial thickness less than the radial distance between saidopposing walls, and coengageable stop means on said bodies for limitingtelescopic extension of said telescopic bodies.
 12. Apparatus forjarring a fish stuck in a well bore: comprising fishing tool meansengageable with said fish, jarring means connected to said fishing toolmeans, a length of drill collars connected to said jarring means,accelerator means connected to said drill collars and having means forconnecting said accelerator means to a pipe string by which said fishingtool means, said jarring means, Said length of drill collars and saidaccelerator means are adapted to be lowered in the well bore to engagesaid fishing tool means with said fish, said accelerator means beingconstructed so that the wave front created upon actuation and release ofsaid jarring means in response to the tension in said accelerator meansand said length of drill collars is reflected downwardly by saidaccelerator means, said accelerator means including a resilientlycompressible elastomeric medium, and means for compressing said mediumupon tensioning said accelerator means.
 13. Apparatus for jarring a fishstuck in a well bore as defined in claim 12, wherein said means forcompressing said medium comprises a compression chamber in saidaccelerator containing said elastomeric medium and a pressure transferfluid for applying pressure to said medium.
 14. Apparatus for jarring afish stuck in a well bore as defined in claim 12, wherein said means forcompressing said medium comprises a compression chamber in saidaccelerator containing said elastomeric medium and a pressure transferfluid for applying pressure to said medium, said medium being composedof rubber having a low bulk modulus.
 15. Apparatus for jarring a fishstuck in a well bore as defined in claim 12, wherein said means forcompressing said medium comprises a compression chamber in saidaccelerator containing said elastomeric medium and a pressure transferfluid for applying pressure to said medium, said medium being composedof rubber having a low bulk modulus and said fluid being a lubricatingoil.
 16. Apparatus for jarring a fish stuck in a well bore as defined inclaim 12, wherein said means for compressing said medium comprises acompression chamber in said accelerator containing said elastomericmedium and a pressure transfer fluid for applying pressure to saidmedium, said medium being composed of silicone rubber.
 17. Apparatus forjarring a fish stuck in a well bore as defined in claim 12, wherein saidmeans for compressing said medium comprises a compression chamber insaid accelerator containing said elastomeric medium and a pressuretransfer fluid for applying pressure to said medium, said medium beingcomposed of silicone rubber, and said pressure transfer fluid being alubricating oil.